1. Field of the Invention
The present invention relates to an arrangement for generating command variables for control loops of a numerically controlled machine, including an interpolator unit and a precision interpolator unit arranged downstream of the interpolator unit.
2. Discussion of Related Art
In numerically controlled machine tools or robots the generation of set points for position, speed and acceleration as the command variables for the control loops of the respective control loops customarily takes place in interpolator units which are arranged upstream of the control loops. For this purpose the desired geometric shaft movement functions and speed profiles are preset at the input of the interpolator units. On the output side, the interpolator unit provides discrete sequences of set points, or scanning values, on a path curve. The set points are output to the downstream located control loop at a defined, customarily fixed interpolator scanning time TIPO as command variables. Before being passed on to the position control loop, the command values generated by the interpolator unit are furthermore additionally filtered in a set point filter, as a rule in the form of digital low-pass filtering, in order to achieve smoothing of the path curve by means of this.
Such an arrangement in accordance with the species is known from International Patent Application WO 01/18622 of Applicant. Measures showing how a suitable position set point filter in the form of an FIR (Finite Impulse Response) filter can be produced with the lowest possible computing outlay are particularly proposed in this application. It furthermore has also been disclosed in this application that it is necessary for the generation of command variables within the time pattern of the control loops to arrange a precision interpolator unit between the set point filter and the control loops, for example the position control loop. The interpolator scanning time TIPO, which typically lies in the range TIPO=[1 . . . 6 ms], or the interpolator scanning rate, are matched to the control loop scanning time TCtrl of the downstream-connected control loop, or to the control loop scanning rate, with the aid of the precision interpolator unit. The control loop scanning time TCtrl lies approximately in the range TCtrl=[100 . . . 800 xcexcs] and is this clearly less than the interpolator scanning time TIPO, or the control loop scanning rate is clearly greater than the interpolator scanning rate. However, details regarding the exact design of the precision interpolator unit cannot be found in this document.
Moreover, a similar architecture of a numerical control has also already been disclosed in the preamble of the specification of DE 43 03 090 A1; however, no suggestions for embodying the precision interpolator unit in a suitable manner can be found in this document, either.
The adaptation of the interpolator scanning time TIPO to the control loop scanning time TCtrl of the downstream-connected control loop is usually achieved by very elaborate methods. For example, in this connection it is known from EP 0 917 033 A2 to perform a so-called polynomial interpolation, or a spline interpolation. However, despite the enormous computing outlay, this type of precision interpolation also provides resultant path curves with undesirable overswings.
Moreover, linear precision interpolation is also employed there. But this method leads to undesirable excitations of the drive systems at the respective segment transitions.
It is therefore an object of the present invention to disclose a practical way in which to produce a precision interpolator unit in connection with an arrangement in accordance with the species for the generation of command variables for control loops of a numerically controlled machine. In this case, the precision interpolator unit should process the position set point generated by an interpolator unit of a defined interpolator scanning rate in such a way that command variables for downstream-connected control loops result within the time pattern of the control loop scanning rate. Demanded are, besides the lowest possible computing outlay for the precision interpolation, the best possible quality of the resultant path curves.
This object is attained by an arrangement for generating command variables for control loops of a numerically controlled machine that includes an interpolator unit for providing position set points with a defined interpolator scanning rate and a precision interpolator unit. The precision interpolator unit includes a scanning rate converter and a downstream-connected low-pass filter, wherein the precision interpolator unit is arranged downstream of the interpolator unit, which generates command variables at an output side from position set points at an input side for one or several downstream-connected control loops, wherein the precision interpolator unit generates command variables in a time pattern of the control loops with a control loop scanning rate.
It is therefore provided in accordance with the present invention to implement the precision interpolator unit with the aid of a scanning rate converter and a downstream-connected digital filter. In this connection, the measure of embodying the digital filter as an FIR filter has been shown to be particularly advantageous. It is possible in this way to combine the running arithmetic operations of the scanning rate converter and of the FIR filter. The input vector of the FIR filter contains a large number of zero values because of the insertion, performed by the scanning rate converter, of intermediate values of the value zero in the time pattern of the control loop scanning rate between the set point values at the input side. Each zero value present at the FIR filter input results in arithmetic operations by the FIR filter whose initial value is again zero. This fact is now used by the present invention, i.e. it is not necessary to perform the operations with an initial value of zero. Instead, for a sequence of input values several interpolated initial values are calculated with the aid of several filter coefficient sets, wherein the number of filter coefficient sets required for this corresponds to the desired interpolation factor. In this way only a small computing outlay by the precision interpolator unit is required.
An excellent quality of the path curve resulting at the end should be mentioned as a further advantage of the arrangement in accordance with the present invention. The reason for this is that an almost exact reconstruction of the output signal is possible by the type of precision interpolation selected, as long as the scanning theorem had not been violated during the interpolation.
Further details of the present invention ensue from the following description of an exemplary embodiment by the attached drawings.